I. Field of the Invention
The present invention relates generally to the fields of immunology, microbiology, and pathology. More particularly, it concerns methods and compositions involving bacterial proteins, which can be used to invoke an immune response against the bacteria. The proteins include proteins of the Ess pathway (e.g., EsaC) and/or peptides or proteins processed by the sortase pathway, including proteins or polypeptides of Staphylococcal and other gram-positive bacteria.
II. Background
The number of both community acquired and hospital acquired infections have increased over recent years with the increased use of intravascular devices. Hospital acquired (nosocomial) infections are a major cause of morbidity and mortality, more particularly in the United States, where it affects more than 2 million patients annually. The most frequent infections are urinary tract infections (33% of the infections), followed by pneumonia (15.5%), surgical site infections (14.8%) and primary bloodstream infections (13%) (Emorl and Gaynes, 1993).
Staphylococcus aureus, Coagulase-negative Staphylococci (mostly Staphylococcus epidermidis), enterococcus spp., Esherichia coli and Pseudomonas aeruginosa are the major nosocomial pathogens. Although these pathogens almost cause the same number of infections, the severity of the disorders they can produce combined with the frequency of antibiotic resistant isolates balance this ranking towards S. aureus and S. epidermidis as being the most significant nosocomial pathogens.
Staphylococcus can cause a wide variety of diseases in humans and other animals through either toxin production or invasion. Staphylococcal toxins are a common cause of food poisoning, as the bacteria can grow in improperly-stored food.
Staphylococcus epidermidis is a normal skin commensal which is also an important opportunistic pathogen responsible for infections of impaired medical devices and infections at sites of surgery. Medical devices infected by S. epidermidis include cardiac pacemakers, cerebrospinal fluid shunts, continuous ambulatory peritoneal dialysis catheters, orthopedic devices and prosthetic heart valves.
Staphylococcus aureus is the most common cause of nosocomial infections with a significant morbidity and mortality. It is the cause of some cases of osteomyelitis, endocarditis, septic arthritis, pneumonia, abscesses and toxic shock syndrome.
S. aureus can survive on dry surfaces, increasing the chance of transmission. Any S. aureus infection can cause the staphylococcal scalded skin syndrome, a cutaneous reaction to exotoxin absorbed into the bloodstream. It can also cause a type of septicemia called pyaemia. The infection can be life-threatening. Problematically, Methicillin-resistant Staphylococcus aureus (MRSA) has become a major cause of hospital-acquired infections.
S. aureus and S. epidermidis infections are typically treated with antibiotics, with penicillin being the drug of choice, whereas vancomycin is used for methicillin resistant isolates. The percentage of staphylococcal strains exhibiting wide-spectrum resistance to antibiotics has become increasingly prevalent, posing a threat for effective antimicrobial therapy. In addition, the recent emergence of vancomycin resistant S. aureus strain has aroused fear that methicillin resistant S. aureus strains for which no effective therapy is available will emerge and spread.
An alternative approach of using antibodies against staphylococcal antigens in passive immunotherapy has been investigated. Therapy involving administration of polyclonal antisera are under development (WO00/15238, WO00/12132) as well as treatment with monoclonal antibody against lipoteichoic acid (WO98/57994).
An alternative approach would be use of active vaccination to generate an immune response against staphylococci. The S. aureus genome has been sequenced and many of the coding sequences have been identified (WO02/094868, EP0786519). The same is true for S. epidermidis (WO01/34809). As a refinement of this approach, others have identified proteins that are recognized by hyperimmune sera from patients who have suffered staphylococcal infection (WO01/98499, WO02/059148).
S. aureus secretes a plethora of virulence factors into the extracellular milieu (Archer, 1998; Dinges et al., 2000; Foster, 2005; Shaw et al., 2004; Sibbald et al., 2006). Like most secreted proteins, these virulence factors are translocated by the Sec machinery across the plasma membrane. Proteins secreted by the Sec machinery bear an N-terminal leader peptide that is removed by leader peptidase once the pre-protein is engaged in the Sec translocon (Dalbey and Wickner, 1985; van Wely et al., 2001). Recent genome analysis suggests that Actinobacteria and members of the Firmicutes encode an additional secretion system that recognizes a subset of proteins in a Sec-independent manner (Pallen, 2002). ESAT-6 (early secreted antigen target 6 kDa) and CFP-10 (culture filtrate antigen 10 kDa) of Mycobacterium tuberculosis represent the first substrates of this novel secretion system termed ESX-1 or 5 nm in M. tuberculosis (Andersen et al., 1995; Hsu et al., 2003; Pym et al., 2003; Stanley et al., 2003). In S. aureus, two ESAT-6 like factors designated EsxA and EsxB are secreted by the Ess pathway (ESAT-6 secretion system) (Burts et al., 2005).
M. tuberculosis variants lacking ESAT-6 (esxA) or CFP-10 (esxB) display severe defects in the establishment of tuberculosis (Guinn et al., 2004; Hsu et al., 2003; Sorensen et al., 1995; Stanley et al., 2003). In S. aureus, failure to produce EsxA and EsxB leads to decreased virulence in a murine abscess model of infection, suggesting that the Ess pathway is involved in the pathogenesis of staphylococcal infections as well (Burts et al., 2005). Thus far, three genes, essA, essB, and essC, appear to be important for production of EsxA and EsxB and possibly secretion across the staphylococcal envelope. The genes are encoded within an eight gene cluster conserved in other Gram positive bacteria (FIG. 1). Of those only esxA, esxB, and essC, share homologues with genes of M. tuberculosis (Burts et al., 2005; Pallen, 2002). The remaining genes in the cluster, esaA, esaB, and esaC, are dispensable for secretion of EsxA and EsxB and are referred to as “accessory” factors for lack of attributable function (esa, ESAT-6 secretion accessory) (Burts et al., 2005).
The first generation of vaccines targeted against S. aureus or against the exoproteins it produces have met with limited success (Lee, 1996). There remains a need to develop effective vaccines against staphylococcus infections. Additional compositions for treating staphylococcal infections are also needed.